This invention relates to the field of cathode ray tube degaussing and, more particularly, to an inexpensive and power-efficient degaussing circuit.
It is well known in the art of CRT's having three electron beams for color reproduction that small amounts of magnetization in the metal parts of the tube can affect the purity of the colors. A very weak magnetic field near an electron beam can produce a deflection error sufficient to cause the beam to strike phosphor dots for which it was not intended. Many types of arrangements have been used in the manufacture of CRT devices for optimizing color purity including locating permanent magnets in or near the convergence yoke assembly, varying the direction of the magnets in a magnetic field equalizer assembly, and adjusting the deflecting yoke axially. Typically, when a CRT device is moved with respect to the direction of the earth's magnetic field, color purity must be readjusted. Degaussing coils have been placed around the periphery of the face plate or around the conical portion of the CRT for demagnetizing portions of the CRT structure which become magnetized during use.
Since energizing of a degaussing coil would cause a noticeable flash on the phosphor screen, degaussing is usually done as the CRT device is turned on and before the electron beam is on. This mode of operation is usually satisfactory for apparatus in the home, such as television sets, but in industrial or business environments there may be relatively strong magnetic fields at frequent intervals due to such things as starting electric motors and the like which severely affect color purity. In some CRT devices, the whole device is shut down manually when degaussing become necessary, but an unwanted shutdown may be less desirable than some color impurity and generally causes loss of data in a data terminal.
Some degaussing control circuits have used a positive temperature coefficient (PTC) resistor in series with the degaussing coil, but since the resistance of a PTC resistor does not become infinite, some current continues to flow in the coil circuit. This current is typically sufficient to keep the resistor warm and, therefore, the degaussing cycle cannot be repeated until the CRT device has been turned off and the resistor allowed to cool. Other circuits utilize combinations of voltage dependent resistors (VDR) and NTC thermistors in order to control the degaussing current. One such circuit puts a second NTC thermistor in series with the first when it is desired to degauss during CRT use but this, too, is expensive.